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How Babies Grow

When does a baby's heart begin to beat? How does she grow? Can he hear my voice? Can she move already?

Fetal development begins soon after conception and typically follows a predictable course. Read on to learn what happens during the first, second and third trimesters. All measurements are approximate.
 
  • The First Trimester: Open or Close
    Weeks 1 and 2
    Conception typically occurs about two weeks after your period begins. To calculate your due date, your doctor will count ahead 40 weeks from the start of your last period. This means your period is counted as part of your pregnancy – even though you weren't pregnant at the time.

    Week 5
    The fifth week of pregnancy marks the beginning of the embryonic period. This is when the baby's brain, spinal cord, heart and other organs begin to form.

    Week 6
    Growth is very fast this week. Just four weeks after conception, your baby's heart is pumping blood. Basic facial features will begin to appear. Your baby's body begins to take on a C-shaped curvature. Small buds will soon become arms and legs.

    Week 7
    Seven weeks into your pregnancy, your baby's brain and face are rapidly developing. Tiny nostrils become visible, and the eye lenses begin to form. The arm buds that sprouted last week now begin to take shape. By the end of this week, your baby is a little bigger than the eraser on a pencil.

    Week 8
    Eight weeks into your pregnancy, your baby's arms and legs are growing longer, and fingers have begun to form. Your baby's ears are forming and your baby's eyes are visible. The upper lip and nose have formed. The trunk of your baby's body is beginning to straighten. By the end of this week, your baby is about 1/2 inch long.

    Week 9
    In the ninth week of pregnancy, your baby's arm bones develop and bend at the elbows. Toes form, and your baby's eyelids and ears continue developing. By the end of this week, your baby is about 3/4 inch long.

    Week 10
    By the 10th week of pregnancy, your baby's head has become more round. The neck becomes defined and your baby's eyelids begin to close to protect his or her developing eyes.

    Week 11
    At the beginning of the 11th week of pregnancy, your baby's head makes up about half of its length. However, your baby's body is about to catch up in the coming weeks.

    Your baby is now officially described as a fetus. Red blood cells are beginning to form in your baby's liver. By the end of this week, your baby's external genitalia will start developing into male or female anatomy. By now, your baby measures about 2 inches long from crown to rump and weighs almost 1/3 ounce.

    Week 12
    Twelve weeks into your pregnancy, your baby is developing fingernails. Your baby sleeps, wakes and exercises its muscles regularly. By now your baby is about 2 1/2 inches long from crown to rump and weighs about 1/2 ounce.

  • The Second Trimester: Find out your baby's sex and feel your baby move Open or Close
    Week 13
    Thirteen weeks into your pregnancy, your baby's intestines have moved into your baby's abdomen. Your baby is beginning to form urine and discharge it into the amniotic fluid. Bone tissue also is developing around your baby's head and within arms and legs.

    Week 14
    Fourteen weeks into your pregnancy, your baby's arms have almost reached the final relative lengths they'll be at birth, and your baby's neck has become more defined. Red blood cells are forming in your baby's spleen. Your baby's sex will become apparent at this time. By now your baby is about 3 1/2 inches long from crown to rump and weighs about 1 1/2 ounces.

    Week 16
    Sixteen weeks into your pregnancy, your baby's eyes have begun to face forward and slowly move. The ears are close to their final position. Your baby can start to make sucking motions with his or her mouth. Your baby's movements are becoming coordinated and can be detected during ultrasound exams. By now your baby is about 4 1/2 inches long from crown to rump.

    Week 18
    Eighteen weeks into your pregnancy, your baby's ears begin to stand out on the sides of his or her head and they begin to hear. By now, your baby is about 5 1/2 inches long from crown to rump and weighs 7 ounces.

    Week 20
    Halfway into your pregnancy, you should be able to feel your baby's movements, also known as quickening. Your baby begins to gain more weight and begins to swallow. By now, your baby is about 6 1/3 inches long from crown to rump.

    Week 24
    Twenty-four weeks into your pregnancy, your baby has developed a schedule of sleep time and wake time. Real hair is growing on his or her head. Fingerprints and footprints are forming. By now, your baby is about 8 inches long from crown to rump and weighs more than 1 1/3 pounds.
  • Third Trimester: Opening Eyes, Gaining Weight and Preparing for Delivery Open or Close
    Week 28
    Twenty-eight weeks into your pregnancy, your baby's eyelids are partially open and eyelashes have formed. Your baby is gaining weight, which is smoothing out many of the wrinkles in the skin.

    By now, your baby is nearly 10 inches long from crown to rump and weighs about 2 1/4 pounds. Babies born this week have a 90 percent chance of survival without physical or mental impairment – and the odds improve with each passing week.

    Week 32
    Thirty-two weeks into your pregnancy, your baby's toenails are visible. Although your baby's lungs aren't fully formed, he or she practices breathing. Your baby's body begins absorbing vital minerals, such as iron and calcium, from the intestinal tract. By now, your baby is about 11 inches long from crown to rump and weighs 3 3/4 pounds.

    Week 40
    Forty weeks into your pregnancy, your baby is about 18 to 20 inches long and weighs 6 to 9 pounds. But remember that healthy babies come in different sizes. Your baby is now full-size and ready to give birth.


Sources: Mayo clinic, American Pregnancy Association, edh.org (videos)
Call us at 916.451.2273, or Text 916.538.1097 or request an appointment online.

This information is intended for educational purposes only and should not be considered professional medical advice. We do not perform abortion services.